ICE Matching Transformer
John Doty
At the risk of permanently damaging my reputation as a cheapskate
tinkerer who'd never actually go out and buy something like this,
I'd like to offer the following:
A Review of the ICE Model 180 Beverage/Longwire Matching Unit
The key to minimizing noise pickup with an end fed antenna (Beverage,
longwire, inverted L, or vertical) is to feed it with coaxial cable
whose shield is well grounded. Unfortunately, coaxial cable is a
poor impedance match to a wire antenna, except perhaps at a few
special "resonant" frequencies. A matching transformer at the base
of the antenna can smooth out the fluctuations in antenna system
efficiency with frequency, yielding an antenna system that works
well enough for good reception from longwave to the top of the
shortwave range.
I've posted instructions for winding such a transformer several
times on rec.radio.shortwave. My homebuilt transformer works well.
However, after about the fifth time this summer that I left the
antenna cable plugged into my receiver by accident, I decided I
needed to improve my lightning protection to minimize the chance
of damage. After looking through catalogs, I decided to buy a
matching transformer with internal lightning protection.
The 180 is *very* solidly built. A barrier terminal strip is provided
for three different antenna inputs, allowing impedance transformation
ratios of 6:1, 9:1, and 12:1 to be chosen. These are labelled as
300, 450, and 600 ohms for use with 50 ohm cable. A "UHF" type coax
connector is provided, along with a nice grounding stud for grounding
the case.
The unit appears to be hand made. The transformer itself is wound
on a two hole "balun core". There's a gas discharge tube across
the transformer secondary and a 3 kV capacitor between the secondary
and the central socket of the coaxial connector. The workmanship
is excellent.
The shunt inductance of the transformer drains static to ground.
The discharge tube clamps the voltage due to a sudden discharge.
Most of the energy in an electrostatic discharge is at low frequency
and below: the capacitor blocks this. This is worthwhile protection,
but not comprehensive enough to completely protect a sensitive
receiver: more protection is needed at the receiver end of the
cable. This is not a criticism: I don't believe that comprehensive
protection is possible in a single package.
The unit is rated for use at frequencies of 1-30 MHz with 50 ohm
cable. I use 75 ohm cable in my system: this might be expected to
reduce the frequency range slightly. To check it, I used the
transmitter for a radio controlled boat (at 27.145 MHz) and a local
radio station (at 1.600 Mhz) as stable (ground wave) reference
signals. I compared the 180, using its 9:1 input, with my homemade
9:1 transformer using the S meter on my Drake R8. The antenna was
a 17 m inverted L. At 27.145 MHz, there was no detectable difference
(<1dB) between the two units. At 1.600 MHz, my homebuilt transformer
yielded a slightly stronger signal (1-2 dB). Nevertheless, the
Model 180 has enough low frequency response that I can easily hear
the antenna's noise floor all the way down to 100 kHz, so a "better"
response would not, in fact, improve my listening.
In short, I cannot find any fault in this unit. At $32 it is also
the least expensive transformer of this type that I'm aware of
(unless you make your own). ICE takes orders at 1-800-423-2666. I
have no connection with ICE except that I'm a satisfied customer.
--
John Doty "You can't confuse me, that's my job."
j...@space.mit.edu